Duration of postpartum anestrus in ewes of the Mexican high plateau

Duration of the postpartum anestrus (DPA) was studied in a typical Mexican flock using 78 ewes, Dorset x Creole and Dorset x Suffolk crosses, that lambed from September to July during the first 10 days of each month. Samples of jugular vein blood (5mL) were taken during the first 90 days postpartum, for determination of progesterone concentration until this reached a level of 1 ng.mL-1 or greater, indicating the end of estrus. The other variables analyzed were phenotype of the ewe, type of parturition (single or twins), parturition number, sex of the offspring, and month of parturition. Data were subjected to analysis of variance using a model of all fixed effects without interaction. DAP was calculated as a function of the period of year in which lambing occurred, least squares means being obtained and compared by Tukey test. Large differences were found between months of parturition (P>.0001), and three periods of year were differentiated by relative DPA, i.e. short (25.90 ± 2.65 days, from June to November); medium (46.9 ± 2.14 days in December and May); long (70.5 ± 2.27 days from January to April). The other variables analyzed did not affect DPA (P < 0.10). These results suggest that DPA varies essentially with month of lambing and that it is possible to distinguish between the minimum physiological DPA and seasoned anestrus.